Dr. Robert Grant in the lab at the Gladstone Institute in San Francisco. Dr. Grant and his team discovered that the drug Truvada is effective against infection of HIV. (Lacy Atkins/The C hronicle)

The Centers for Disease Control and Prevention said Wednesday that Americans at risk for HIV should take a daily pill to prevent infection, a recommendation that could significantly drive sales for the drug’s maker, Gilead Sciences.

The Foster City company’s Truvada costs $15,000 a year, and is the only preventative therapy of its kind on the market. It has a potentially vast audience: About 50,000 Americans become newly infected with HIV every year, a rate that, despite health outreach efforts, has remained virtually the same since the mid-1990s. And most insurers cover the drug.

Mid-day Thursday, Gilead’s stock was down about 1.4 percent from the day before, to $79.84, but the company’s stock has been struggling along with other biotechnology stocks for the last few months.

It’s another big stroke of fortune in a short amount of time for Gilead. The company has already been breaking records with its best-selling new hepatitis C drug, Sovaldi, which racked up $2.3 billion in worldwide sales in the first three months of the year.

Truvada is a PrEP (pre-exposure prophylaxis) drug intended for people who test negative for HIV but are considered at risk, such as gay men who have sex without condoms, patients who regularly have sex with anyone they know is infected, and anyone who shares needles or injects drugs. Truvada is supposed to be used in combination with safe-sex practices, such as condom use, to help prevent risk — not alone. But some health experts have worried that some people using the drug might engage in riskier behavior because they believe they are protected.

(UPDATED: This post has been changed to correct the price of Truvada and the size of its potential market.)